The present invention relates to the medical imaging arts. It particularly relates to dynamic volumetric cardiac imaging using computed tomography (CT), and will be described with particular reference thereto. However, the invention will also find application in conjunction with the dynamic volumetric imaging of other non-stationary organs, such as the lungs, in conjunction with time evolution studies, and in conjunction with other imaging techniques and modalities.
It is well known that cardiac diseases are a leading health problem in the United States at the present time. In spite of this prevalence, however, medical diagnosis of cardiac diseases is inconsistent. More than half of the individuals who die of a coronary attack do not exhibit previously recorded symptoms. Hence, there is a need for improved screening and accuracy in the diagnosis of coronary problems.
Medical imaging techniques can image and monitor selected heart functions, such as blood flow or cardiac muscle motion, in real time and without disturbing the heart activity. These techniques are commonly used in diagnosing and monitoring cardiac diseases. However, the existing cardiac imaging modalities, including cardiac angiography, ultrasound, magnetic resonance imaging (MRI), and various forms of computed tomography (CT), are not fully satisfactory. Cardiac angiography is an invasive technique. Ultrasound suffers from poor image quality. MRI typically does not detect calcification, and is subject to various artifacts.
Recent improvements in the speed and resolution of spiral/helical cone-beam and multi-slice CT imaging scanners have led to increased interest in dynamic volumetric CT imaging. Typically, dynamic volumetric cardiac CT imaging includes synchronization with the cardiac cycle using an electrocardiograph (ECG) in a technique known as ECG gating. In prospective ECG gating, detection of a selected ECG waveform or other trigger event controls initiation of CT imaging. In retrospective ECG gating, both the ECG signal and simultaneously acquired CT imaging data are stored in a memory, and the CT image reconstruction is subsequently performed with reference to timing information contained in the stored ECG signal. In a usual approach, ECG-gated cardiac CT imaging data is acquired around a selected cardiac phase of the cardiac cycle, where the cardiac phase is defined as a temporal position within the cardiac cycle.
However, cardiac CT imaging, including cone-beam CT imaging, suffers from motion blurring due to the relatively long CT image scan times needed to acquire projection data with at least 180° of angular coverage. This large angular coverage is typically used to ensure adequate sampling to reconstruct the entire field of view. Obtaining this angular data range involves imaging over a substantial portion of the cardiac cycle.
A typical modern high-speed CT scanner operates at about 120 x-ray source revolutions per minute corresponding to 500 milliseconds per revolution, so that a 180° angular scan takes about 250 milliseconds. A typical heart pulse rate of 70 beats per minute corresponds to a cardiac cycle period of 857 milliseconds. Hence, the imaging occurs, not at a single well-defined cardiac cycle phase, but rather over a substantial fraction of an entire cardiac cycle, i.e. over almost 30% of the cardiac cycle in the exemplary case. The motion of the cardiac muscle as data is acquired over this extended time causes motion-related image blurring. The high pulse rates often characteristic of patients with coronary diseases further increases motion image blurring.
To improve temporal resolution, it is known to acquire projection image data over two or more cardiac cycles while simultaneously monitoring the cardiac cycle using ECG gating. Data at the selected cardiac phase is extracted and combined from the two or more cardiac cycles based on the cardiac phase determined by the ECG. By distributing the imaging over two or more cardiac cycles, the data acquisition period within each cycle is shortened, which improves temporal resolution with respect to the cardiac cycling because a smaller portion of the cardiac cycle is sampled.
As more cardiac cycles are sampled, the data acquisition period within each cycle becomes shorter and the temporal resolution with respect to the cardiac cycling improves. However, collecting imaging data over more cycles can introduce other artifacts. First, patient motion is more likely to occur as the number of sampled cardiac cycles (and hence the total imaging time for a reconstructed view) increases.
Additional artifacts can be introduced by variations in the cardiac cycle period (i.e., the pulse rate) between the several sampled cardiac cycles. Data is acquired at a cardiac phase corresponding to a selected temporal position within the cardiac cycle. It is known in the art to use a cardiac phase defined as a percentage temporal position within the cardiac cycle to account for changes in the cardiac cycle period. However, the various physiological portions of the cardiac cycle, e.g. the systolic and diastolic portions and physiological events therein, do not linearly scale with changes in the cardiac cycle period, and so using a percentage temporal position does not fully compensate for changes in the pulse rate.
Third, image artifacts can be introduced in helical cone-beam CT due to an introduction of potential inconsistencies in the angular deviation in the Z-direction (i.e., the cone angle) at segment boundaries. In helical cone-beam CT, the x-ray beam diverges in the Z-direction and the patient is linearly moved or scanned in the Z-direction during the imaging. The cone angle of the x-rays in the Z-direction varies continuously during continuous helical scanning. However, when discontinuous data segments are combined to form a complete data set, there can be inconsistencies in the cone angle at the data segment boundaries. Although the data can still be combined to produce a complete data set of 180° or greater angular coverage, potential cone angle inconsistencies in the Z-direction at the data segment boundaries can produce artifacts in the reconstructed image.
Another issue that arises in ECG-gated CT performed over several cardiac cycles is coordinating between the angular rotation of the x-ray source and the cardiac cycle such that data obtained from successive cardiac cycles is complementary rather than partially redundant. The data preferably combines to generate a continuous data set over the desired angular coverage, e.g. 180°; however, this is not always the case.
Two undesirable limiting cases exemplify the coordination issue. In the first limiting case, the cardiac cycle period is an integer multiple of the gantry rotation period, i.e. Tcc=nTrot where n is an integer, Trot is the gantry rotation period, and Tcc is the cardiac cycle period. Under this condition, for any temporal window triggered by a selected cardiac phase the same angular span of CT projection data will be acquired in successive cardiac cycles. This data is not complementary and does not provide increased angular coverage versus the data acquired during a single cardiac cycle.
In the second limiting case, the period relationship is Tcc=(n+½)Trot. The gantry angles in this case will be 180° apart at any particular selected cardiac phase in any two successive cardiac cycles. Redundant data is collected in alternate cardiac cycles. The two acquired data sets will be angularly displaced by 180°, with a significant angular gap between the two data sets, which produces image artifacts. In cardiac ECG-gated CT, angular data portions are preferably combined into a contiguous but substantially non-overlapping data set.
In one previous approach to this problem, the cardiac period is estimated using an ECG, and the CT scanning rate is synchronized with the estimated cardiac cycle. However, this method is susceptible to problems due to changes in the cardiac cycle period between the calibration and the imaging, or during the imaging. Some variation in the cardiac cycling is to be expected since the examined patient is usually in an anxious state and is being asked to hold his or her breath over the course of the image acquisition. Also, patients having coronary disease often suffer from heart arrhythmia wherein the cardiac cycle is unpredictably non-periodic. Another drawback to this method is that the CT scanning rate is reduced to correspond with the heart rate. Hence, the CT apparatus is typically being operated below its rated capabilities (e.g., 120 rpm) when using this method.
The present invention contemplates an improved method and apparatus for imaging dynamically moving organs which overcomes the aforementioned limitations and others.